RELIABILITY-BASED DESIGN RULES FOR THIN-WALLED STRUCTURAL STEEL BEAM-COLUMN ELEMENTS
RELIABILITY-BASED DESIGN RULES FOR THIN-WALLED STRUCTURAL STEEL BEAM-COLUMN ELEMENTS
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Date
1994-02
Authors
AFOLAYAN, Joseph Olasehinde
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Abstract
The problem considered in this study is to
deterministically and probabilistically evaluate the adequacy
of the design methods of cold-formed structural steel beamcolumns
having doubly-symmetric cross-sections in accordance
with the AISI Specification of 1968 and ECCS Recommendation of
1987 with a view to proposing equivalent optimal criteria. The
validity of the design formulae are verified using test results
drawn from literature and those observed at the Heavy
Structural Laboratory at the University of Dortmund, Dortmund,
West Germany. Depending on the ratio of width to thickness of
wall members, varying degrees of incongruities exist between
the predictions given by the design formulae and test results.
Both cases of good and over-estimation of member strength are
noted and generally the predictions of ECCS (1987) show more
conservatism than AISI (1968). Due to scatter in test results,
the influence of the statistics of the design variables on the
predicted strength is analyzed. The sensitivity is compared
giving recognition to the parameter, Qw, which represents the
local buckling phenomenon. The design formulae are found
sensitive to this parameter.
Safety factors are applied either to loading or strength
parameters in the two design equations studied. The
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dependability of such factors is measured by computing the
reliability levels implicit in the design equations under
uncertain dead and maximum life-time live loads. Within the
range of practical data points considered, significant
inconsistency is noted. Although the reliability levels
computed assume random values, their distributions show that a
target level is in focus. Thus, employing a socio-economic
criterion, a target reliability level, 82, of 4.265 is obtained
for each design equation. This value is equivalent to a failure
probability level of 1 x 10-5.
Since an ultra safe design method can be unduly expensive
and that it is not economically feasible to design a structure
that cannot fail, resistance and load factored Limit State
Design Formats which trade-off safety and economy are derived
as alternatives to the corresponding formulae in AISI (1968)
and ECCS (1987). These formats are optimal having a minimum
total cost implication as far as the design of thin-walled
structural steel beam-column members of doubly-symmetric shapes
or shapes not subject to torsional or torsional-flexural
buckling is concerned. In addition, they supercede the current
formats by providing uniform reliability level at minimum cost.
This objective became realistic as a consequence of rigorous
combination of the Advanced First-Order Reliability Method
(FORM) and a Nonlinear Quadratic Programming Technique.
Description
Dissertation presented to the Postgraduate School,
Ahmadu Bello University, Zaria, in conformity with the General
Requirements for the Degree of Doctor of Philosophy (PhD)
in Structural Engineering.
February, 1994
Keywords
RELIABILITY-BASED DESIGN RULES,, THIN-WALLED STRUCTURAL STEEL,, BEAM-COLUMN ELEMENTS